I installed my deck hardware 7 years ago, and no leaks as of yet!
I have a foam cored deck, and dug out the foam core around the fitting holes using an allen wrench. First I drilled the hole through the top deck layer and through the foam, but not through the inner fiberglass layer. Previously I prepared my reaming out tool - an allen key - I sharpened the short end into a knife and put the long end in a drill. Then I inserted the knife end into the hole and ran the drill. Bits of foam came out the hole. I vacuumed the bits out of the hole and filled it with epoxy. After it set I re-drilled the fitting hole all the way through. Now I can't crush the deck and if the fitting leaks it doesn't penetrate into the foam. I used life-calk on the threads and under the fittings. I tightened once and have had no leaks at all on the hundreds of holes needed to set all the fittings.